1. Field of Invention
The present invention relates to a diagnostic technology for an emission control apparatus that lessens undesired emissions from an internal combustion engine and, more particularly, to a technology for diagnosing a fault or deterioration of an adsorbent that adsorbs unburned exhaust gas components.
2. Description of Related Art
Many internal combustion engines of, for example, motor vehicles, have in the exhaust system thereof a catalyst device in which precious metals, such as platinum, palladium and the like, are supported as catalysts in order to significantly lessen harmful exhaust gas components, for example, carbon monoxide (CO), nitrogen oxide (NOx), hydrocarbons (HC) and the like, before passing the exhaust into the atmosphere.
A typical catalyst device of the aforementioned kind causes HC and CO present in exhaust gas to react with O.sub.2 present in exhaust gas thereby oxidizing HC and CO into H.sub.2 O and CO.sub.2, and causes NOx present in exhaust gas to react with HC and CO present in exhaust gas thereby reducing NOx into H.sub.2 O, CO.sub.2 and N.sub.2.
At the time of startup of an internal combustion engine, the engine air-fuel ratio is set lower than the theoretical air-fuel ratio (that is, shifted to the fuel-rich side) in order to facilitate the startup of the engine. Furthermore, relatively low engine temperature during startup causes unstable combustion. Therefore, unburned gas components, such as unburned hydrocarbons and the like, are discharged in relatively large amounts at the time around engine startup.
The catalyst device of an internal combustion engine becomes able to significantly lessen the harmful exhaust gas components when the temperature of the catalyst device reaches or exceeds a predetermined activation temperature. Therefore, when the activation temperature has not been reached, for example, at the time of cold start of the engine, the catalyst device cannot sufficiently lessen unburned gas components, which are discharged in large amounts in such a situation.
As a countermeasure against the aforementioned problem, an emission control apparatus has been proposed in which an adsorbent that adsorbs unburned gas components when the adsorbent is below a predetermined temperature and that releases the unburned gas components when the adsorbent has reached or exceeded the predetermined temperature, is provided upstream of a catalyst device.
In this emission control apparatus, the adsorbent adsorbs the unburned gas components when the catalyst device has not activated. After the catalyst activates, the catalyst device lessens the unburned gas components desorbed (i.e., released) from the adsorbent and also lessens undesired emissions in the exhaust from the internal combustion engine.
However, if the adsorbing capacity of the adsorbent decreases due to a fault, deterioration or the like, the adsorbent becomes unable to adsorb the entire amount of the unburned gas components present in exhaust gas, and passes portions of the unburned gas components out into the atmosphere, thereby degrading the emission quality.
Therefore, in order to prevent degradation of emissions due to an abnormality of the adsorbent, it is important to detect a fault, deterioration or the like of the adsorbent with high precision.
Japanese Patent Application Laid-Open No. HEI 8-121232 discloses an HC adsorbent deterioration diagnostic apparatus. This deterioration diagnostic apparatus has air-fuel ratio sensors that are provided upstream and downstream of the HC adsorbent for detecting exhaust air-fuel ratios. At a time at which the HC adsorbent should release unburned gas components, the diagnostic apparatus determines whether the HC adsorbent has deteriorated, on the basis of the difference between an output signal of the upstream air-fuel ratio sensor and an output signal of the downstream air-fuel ratio sensor, or a quantity corresponding to the output difference.
This diagnostic technology is based on the finding that if the HC adsorbent is functioning normally, the value of the output signal of the downstream air-fuel ratio sensor shifts from the value of the output signal of the upstream air-fuel ratio sensor to the fuel-rich side by a deviation corresponding to the amount of the unburned gas components desorbed from the HC adsorbent. When the difference between the value of the output signal of the downstream air-fuel ratio sensor and the value of the output signal of the upstream air-fuel ratio sensor becomes less than a predetermined value, it is determined that the adsorption performance or the desorption performance of the HC adsorbent has deteriorated.
However, if the flow of emissions from the internal combustion engine changes so that the amount of unburned gas components desorbed from the adsorbent sharply increases, the air-fuel ratio of exhaust gas flowing downstream of the adsorbent may become an excessively rich ratio that is outside the detection range of the air-fuel ratio sensor. In such a case, precise fault diagnosis may become impossible.